In the treatment of diseases, injuries or malformations affecting spinal motion segments (which include two adjacent vertebrae and the disc tissue or disc space between them), and especially those affecting disc tissue, it has long been known to remove some or all of a degenerated, ruptured or otherwise failing disc. In cases in which intervertebral disc tissue is removed or is otherwise absent from a spinal motion segment, corrective measures are indicated to insure the proper spacing of the vertebrae formerly separated by the removed disc tissue. Commonly, the two vertebrae are fused together using transplanted bone tissue, an artificial fusion element, or other compositions or devices. Fusion of adjacent vertebrae is a procedure that is performed many tens of thousands of times each year in the United States and Canada.
There is increased concern in the spinal medical community, however, that the biomechanical rigidity of the intervertebral fusion may predispose neighboring spinal motion segments to rapid deterioration. See, e.g., B. H. Cummins, J. T. Robertson, and S. S. Gill, Surgical Experience With An Implanted Artificial Cervical Joint, J. Neurosurg. 88:943-948, 1998. For example, removal of the intervertebral disc and subsequent fusion of the C3-C4 vertebrae may spur deterioration of the C2-C3 and C4-C5 segments. A spinal fusion prevents the fused vertebrae from moving rotationally or translationally with respect to each other, as natural disc tissue permits. That lack of mobility may increase the stresses on adjacent spinal motion segments. Follow-up of patients undergoing a successful fusion may reflect an incidence of neighboring segmental disease as high as 20%, and indications are that 50% of fusion patients will continue to have complaints of pain. Several conditions have been found at spinal segments adjacent to a spinal fusion site, including disc degeneration, disc herniation, instability, spinal stenosis, spondylosis and facet joint arthritis. Consequently, many patients may require additional disc removal and/or fusion procedures as a result of a spinal fusion. Alternatives to fusing a spinal motion segment therefore clearly have inherent advantages.
Several different types of intervertebral disc arthroplasty devices have been proposed for preventing collapse of the space between adjacent vertebrae, to reduce pain and to maintain stability and range of rotational motion between the vertebrae. For example, U.S. Pat. No. 5,755,796 discloses a prosthesis for implantation in the disc space between adjacent vertebrae. The prosthesis has two elements, one being attached to the vertebra above the disc space, and the other being attached to the vertebra below the disc space. The lower element includes a hollow box frame having a spherical seat inside, and the upper element includes a spherically-shaped head that fits into the spherical seat. U.S. Pat. No. 5,556,431 discloses a three-piece intervertebral disc endoprosthesis, in which two plates, each having a hollow in one side, are fixed to adjacent vertebrae so that the two hollows face each other across the disc space. The third element is a core having an outwardly rounded surface on each side that fits into the hollows in the vertebral plates. Similarly, U.S. Pat. No. 5,684,296 discloses a prosthesis having two L-shaped members, each having curved sections between which a resilient disc body is placed.
U.S. Pat. Nos. 5,782,832 and 5,683,465 disclose implants having two plate members anchored to adjacent vertebrae within the disc space. In U.S. Pat. No. 5,782,832, the upper plate member has a rounded underside and the lower plate member has a hollowed top side to engage the underside of the upper plate member and allow rotation. In U.S. Pat. No. 5,683,465, the plate members have a snap-fit ball and socket engagement allowing rotation. Other prosthetic intervertebral discs are also known. Some are composite components having metal and rubber or polymer inserts which are designed to replicate the characteristics and performance of the discs in separating and enabling movement of adjacent vertebrae with respect to each other. Other designs are more mechanical in nature, including springs or other damping mechanisms also designed to approximate the behavior of a normal disc.
The normal movement of vertebrae with respect to one another involves variable rotation and translation. These components of the movement are dictated by the shape and size of the individual vertebrae including the relative positions of the unconvertebral joints (in the cervical spine), the orientation of the facet joints and ligamentous constraints. In essence the axis of rotation for any two vertebrae is unique and during flexion or extension will shift, or translate.
The prior art describes disc prosthesis or joints that dictate a fixed axis of rotation that will not permit translation and will therefore prevent normal movement and tend to jam. The latter may lead to excessive stresses on the prosthesis leading to wear or failure or be transferred to adjoining motion segments provoking pain and/or accelerated degeneration.
Further, many prior devices are relatively difficult to implant. Most are designed to be attached via integral anchors to the vertebral surface facing the intervertebral disc space. To implant such a device, the adjacent vertebrae must be spread substantially further apart than their normal distance from each other, so that the prosthesis can be maneuvered between the vertebrae and the anchors can be inserted into their respective vertebrae. Such an operation presents additional risk of injury to the vertebrae themselves from misplacement or scratching by the anchors, and to other tissues due to the spreading of the vertebrae beyond its normal span.
Therefore, there remains a need for an intervertebral joint prosthesis allowing translational as well as rotational movement between implanted vertebrae, and which is easy to implant in a disc space. There is also a need for a method and instrumentation for implanting such an intervertebral joint prosthesis.